Typically, non-destructive radiographic techniques are used to radiograph the weldments of large diameter high pressure vessels with thick walls without access to the interior of the vessel. The present radiographic procedures demand unacceptably long exposure times in order to meet the testing standards set forth by ASME, Section V. Such long exposure times require that the pressure vessel be kept out of service for a long period of time. Additionally, the restricted area (surrounding the radiographic procedure) necessary for personnel is 1,257 feet. This safety requirement impacts the normal operation where the vessel is located.
The use of radioisotopes to provide the gamma ray source for radiographic examination presents a problem in that the isotope sources present a severe radiation hazard and when not in actual use must be handled carefully and stored and locked in shielded containers. Thus, remote handling of the gamma ray source is necessary and such remote handling requires that the gamma ray source (cobalt 60, in the present application) be first moved by remote controls) from the center to the surface of a shielded container and then to a point some distance away. In the present application, this point is a collimator mounted adjacent the pressure vessel to be tested.
The collimator of the present invention is designed to support and shield Co 60 during actual radiographic exposure and to focus the radiation from the Co 60 onto the weldment of the pressure vessel. The collimator allows the radiation to be emitted in a uniform path toward through the point (surface) being tested toward a film which is used to record the amount of radiation passing through the tested surface. The collimator is designed to shield the Co 60 in such a manner as to prevent an excessive amount of radiation from being emitted in unwanted directions. An additional benefit provided by the instant collimator is the mitigation of undesirable backscatter radiation which adversely affects the quality of radiographs thus, permitting the radiographer to produce radiographs with excellent quality and sensitivity. Another feature of the collimator of the present invention is that it reduces the restricted safety area by 95%. Example: safe stand-off distance 61 feet with the collimator of the present invention versus 1,256.7 feet with no collimator. Additionally, the present collimator greatly reduces the radiographic exposure time of presently used radiographic equipment when used in conjunction with a special technique under development.
It is an object of the present invention therefore, to provide a gamma-ray collimator for non-destructive testing of pressure vessels or the like.
It is a further object of the present invention to provide such a collimator for supporting and shielding a radioisotope source during the non-destructive testing procedure.
It is another object of the present invention to provide such a collimator with means for focusing the radiation in a predetermined uniform path to the target.
It is yet another object of the present invention to provide such a collimator which prevents excessive backscattering of radiation and thus enhances the quality and sensitivity of the radiographs.